The present invention relates generally to ceramic materials and specifically to such materials and their preparation to achieve increased density and thermal shock resistance.
Ceramic materials are increasingly becoming an integral part of modern industry. Applications of novel advanced ceramic materials can provide significant economies, increase productivity and expand product markets. Industry requires materials which have both high strength at room temperature and high strength at elevated temperatures that resist deformation, are damage tolerant, and that resist cracking, corrosion and erosion where thermal stress and mechanical stress is a factor. Usually the thermal shock resistance of high density ceramic materials is relatively low. New materials are being developed worldwide using a number of principles relating to different-compositions and several different structural characteristics, including atomic, electronic, grain boundary, micro-structure, and macro-structure.
Certain ceramic products are desirably fabricated from high purity materials, e.g., materials that are substantially entirely comprised of a particular ceramic component or material. Such materials are polycrystalline in structure and are utilized for such products as the fabrication of cutting tools and other applications where wear resistance is an important property and capability. Representative techniques for the preparation of such high density materials include those set forth in U.S. Pat. No. 5,342,564 to Wei et al. Wei et al. propose to prepare alumina-titanium carbide composites of high density by a specific rapid sintering regime which heats the green body in question to a temperature up to which thermal shock is experienced. Generally, however, improvements in properties such as thermal shock resistance and fracture toughness can only be achieved, if at all, by substantial doping with other ceramic or refractory materials such as titanium carbide. Moreover, the process so disclosed is careful to control heating so as to avoid thermal shock of the green body, and clearly, is neither expected to achieve nor actually achieves any improvements in these properties.
One of the drawbacks in the processing of high density ceramic materials of the type described is the development of extensive crack propagation and corresponding susceptibility to fracture in use. For example, the fabrication from ceramic materials of metal casting sleeves of the type regularly utilized in the fabrication of automotive parts, involves a highly critical and correspondingly expensive fabrication owing to the tendency of the dense body to develop unwanted cracks and fractures in fabrication. Generally, such sleeves are first fabricated, then cooled to shrink-fit them into the casings into which they are disposed for use. In use, the bodies experience extreme temperature fluctuations and because of their high density and corresponding susceptibility to thermal shock damage, must be frequently replaced, at great cost to the manufacturer in terms of equipment and downtime. At the same time, however, the density of materials utilized as shot sleeves must necessarily be high, and, in fact, as close to theoretical as possible, so that these materials will withstand the abrasive forces of a continuous casting process.
U.S. Pat. No. 3,887,524 to Kirchner et al. seeks to prepare an alumina body with a combination of improved strength and thermal shock properties, and employs a quenching procedure where the body is fired up to temperatures on the order of 1750xc2x0 C. and is then quenched in a liquid quenching medium held at a temperature below about 250xc2x0 C. This specific quenching medium is an emulsion of oil with water, and several oils are proposed and illustrated. The materials prepared in accordance with Kirchner et al., however, did not purport to exhibit the high density characteristics of products of interest herein and, in fact, by the flexural strength properties revealed, would not be expected to so perform. This same problem is reflected in U.S. Pat. No. 5,139,979 to Anderson et al. where aluminum titanate composites are prepared, which are purported to offer the properties of improved thermal shock resistance, high mechanical strength and low coefficient of thermal expansion. As reflected in Table 4 of Anderson et al., materials prepared with the described properties set forth by the patentees exhibited densities that were substantially lower than theoretical. This further evidences the understanding in the art that the properties of high density and improved thermal shock resistance are generally incapable of conjoint achievement in a single ceramic body.
As both properties are desirably achieved, and particularly inasmuch as the combination of these properties would yield a material having a broad range of commercial and other industrial applications, a need is therefore perceived to exist which is believed to be fulfilled by the present invention as set forth below.
In accordance with the present invention, a ceramic material is disclosed which exhibits a combination of properties not found in any known ceramic materials. Specifically, the ceramic materials of the present invention exhibit a combination of extremely high density approaching theoretical levels, together with an increased modulus of elasticity and, notably, increased thermal shock resistance as measured by the ability of the ceramic material to withstand a change in its temperature of 300xc2x0 C. or greater, as is caused by the rapid upquenching of said ceramic material, without the development by the ceramic material or a structure formed from it, of extensive fracture or like structural degradation. Exemplary ceramic materials include alumina, zirconia, titania, thoria, silica, magnesia, calcia, nitrides thereof, carbides thereof, aluminum nitride, silicon nitride, boron nitride, boron carbide, and mixtures thereof, with alumina, aluminum nitride and silicon nitride being particularly exemplary. In a particular embodiment, the ceramic material comprises alumina, and more particularly, alumina of 85%, 96% and 98.8% purity, respectively. As illustrated herein, densities on the order of 3.8 g/cm3 and thermal shock resistance of 650xc2x0 C. or greater are attained by the within invention. The invention also extends to composites prepared with materials such as alumina, zirconia, magnesia, silica and calcia. Also, mixtures of materials are contemplated, so that, for example, composites prepared to contain 75% to 90% alumina, remainder zirconia, are included.
A ceramic structure or product in accordance with the invention is prepared by a process which comprises, in a first embodiment:
(A) forming the ceramic structure;
(B) applying a pressure or stress to the ceramic structure of Step (A) on the order of 50,000 psi as determined in relation to the size and shape of said ceramic structure, and a particular K factor relating to modulus of elasticity;
(C) concurrently with step (B), adjusting the temperature of said ceramic structure to a level on the order of 650xc2x0 C. or other source of energy of activation up to and exceeding the elastic limit of said material, as determined in relation to alumina and to a particular K factor relating to the heat capacity, thermal conductivity and thermal expansion of the ceramic structure; and
(D) performing the treatment in accordance with the conditions of Steps (B) and (C) at a frequency of at least once and for a period sufficient to reach temperature equilibrium within said ceramic structure, and to thereby achieve the pressure parameter of at least the magnitude of Step (B).
A similar process that may be practiced in accordance with the method of preparation of the present invention comprises:
(A) providing a ceramic structure and subjecting it to at least one first heat treatment to promote the formation of controlled macro- and micro-fracture domains therein; and
(B) subjecting the ceramic structure treated in accordance with Step (A) to at least one second heat treatment while placing said ceramic body under tangential static pressure to cause the conversion of the ceramic body to an anisotropic polycrystalline form.
The method of the last mentioned embodiment comprises a first heating of the initial ceramic body at a temperature of up to about 1200xc2x0 C., followed by a cooling of the heat treated body to ambient, or to a temperature of about 20xc2x0 C. Both heating and cooling may desirably be performed rapidly and, for example, may be conducted at a rate as high as 100xc2x0 C. per second. Heating may take place in air, while cooling may be conducted by means of a liquid medium, such as water. In such event, the cooled body may thereafter be dried.
The second stage or step of the process involves the application of heat and pressure which is believed to result in the change of the crystal structure of the ceramic material. Specifically, Step (B) of the process involves the heating of the ceramic body to temperatures of up to about 1200xc2x0 C. while applying pressure or stress, and more particularly, stress in amounts ranging from about 10,000 psi to about 280,000 psi, and more particularly, from about 60,000 psi to about 150,000 psi, and most preferably, at a level of at least 90,000 psi. The performance of this second treatment step is believed to yield a change in the crystal structure of the resulting ceramic body not previously observed. More importantly, the resulting ceramic body develops the optimal density, strength and thermal shock resistance reflective of the present invention.
Both steps of the process may desirably be performed a plurality of times, and particularly the second step has been found to yield favorable improvements in all desired properties by such repeated performance.
The invention also extends to the treatment of previously formed ceramic structures and products, in which embodiment the below procedure may be followed:
(A) applying a stress or pressure to said ceramic structure, on the order of 50,000 psi as determined in relation to the size and shape of said ceramic structure, and a particular K factor relating to modulus of elasticity;
(B) concurrently with step (A), adjusting the temperature of said ceramic structure to a level on the order of 650xc2x0 C. or other source of energy of activation up to and exceeding the elastic limit of said material, as determined in relation to alumina and to a particular K factor relating to the heat capacity, thermal conductivity and thermal expansion of the ceramic structure; and
(C) performing the treatment in accordance with the conditions of Steps (A) and (B) at a frequency of at least once and for a period sufficient to reach temperature equilibrium within said ceramic structure, and to thereby achieve the pressure parameter of at least the magnitude of Step (A).
In a yet further embodiment of the present invention, the method for the preparation of a ceramic body having the favorable combination of high density, increased modulus of elasticity and increased thermal shock resistance is prepared by a first heating step wherein the ceramic body is exposed to flash heating to a temperature of about 675xc2x0 C., flash cooling of the same, and subsequent heat treating of the material for stabilization at a temperature that may range from about 1100 to about 1300xc2x0 C. The thus treated ceramic body may then be placed under a pressure of at least 90,000 psi while being subjected to thermal cycling at a temperature of up to about 660xc2x0 C., to confer the final favorable combination of properties on the resulting ceramic body.
The repetition of the process which may be modulated and monitored by intermittent measurement of the ceramic body to ascertain its properties may be achieved at predetermined levels. The rapid upquenching referred to herein and in the claims may be effected by the exposure of the surface of the ceramic material to contact with eg. molten aluminum, in the same fashion as when molten metal is poured against the surface of a ceramic shot sleeve during a metal casting operation; In this connection, the invention extends to articles useful in metal casting procedures, such as the cylindrical shot sleeve that is conventionally employed in engine fabrication.
Likewise, the imposition of stress as is practiced in accordance with the present invention achieves the favorable development of anisotropic properties in the resulting body, that further advance other favorable characteristics as electrical conductivity and the like. Among the properties that are exhibited by the ceramic bodies prepared in accordance with the present invention are a reduction in crystal volume together with increases in thermal shock resistance and in the number of micro-cracks per unit volume and their homogeneity of distribution, as well as increased density and improved corrosion, erosion and abrasion resistance.
Thus, ceramic bodies produced in accordance with the present invention have a full array of favorable properties that commends their application to favorably diverse product areas. For example, the ceramic bodies of the present invention may be utilized for aluminum die casting or extrusion; chemical manufacturing, and specifically, ingredients for the preparation of chemical composites and equipment used in chemical processing where, particularly, inert materials are desirably used to reduce contamination of endproducts; aerospace applications, such as heat shields, turbine blades, seals, nozzles, and like structures; medical and scientific products, including manufacturing equipment for the production of pharmaceutical products as well as prosthetic devices, scientific instrument components and laboratory hardware; electronic and electrical products, such as packaging of integrated circuits, electro-optical devices, piezoelectric devices, substrates, resistors, thermistors and the like; and ceramic molding and cutting tools. In each instance, the particular characteristics of the resulting body may be predetermined and explicitly achieved to maximize the applicability of the resulting body to suit the specific industrial application.
More particularly, the invention extends to the preparation of cylinders for use in the manufacture of, e.g., internal combustion engines, such as for automobiles, trucks, industrial equipment, and other applications where motive power of this type is employed. The manufacture of an internal combustion engine often involves the casting of the primary components such as the engine block. In turn, the casting operation of the engine block includes the definition of the cylinder chamber by placing a cylindrical piece inside the mold against which the metal will be cast. This cylindrical piece is conventionally prepared from a ceramic composition that must exhibit sufficient density and thermal shock resistance to maintain its dimensional stability and integrity through repeated casting procedures. The advantage of the present invention is that inserts prepared hereby exhibit significantly improved stability and useful life.
The invention accordingly extends to a cylindrical body or insert for use in the manufacture of an internal combustion engine, which cylindrical insert is prepared from a ceramic material exhibiting a combination of near-theoretical density, increased modulus of elasticity and increased thermal shock resistance as measured by the ability to withstand a temperature difference greater than 300xc2x0 C.
A further advantage of the ceramic materials of the invention that has been noted generally is that their improved thermal shock resistance yields a corresponding improvement in electrical properties, such as, for example, improved electrical loss factor and capacitance. Accordingly, the present ceramic materials are particularly useful for the preparation of ceramic structures which include integrated circuits. Such structures are also known as microchips, and the present materials may therefore be used in ceramic packaging manufacture.
The present method may be rapidly performed within, for example, a matter of hours, to achieve the ceramic bodies having the desired combination of properties.
Accordingly, it is a principal object of the present invention to provide a ceramic body having a desirable combination of high density and high resistance to thermal shock damage.
It is a further object of the present invention to provide a ceramic material as aforesaid that may be particularized as to its properties during processing with predictability and economy.
It is a yet further object of the present invention to provide a ceramic material as aforesaid that offers a diversity of favorable properties commending broad usage in a variety of industrial and technical applications.
It is a still further object of the present invention to provide a method for the preparation of a ceramic body as aforesaid, which method is capable of rapid and economic practice.
It is a still further object of the present invention to provide a variety of products having broad industrial applications, by fabrication from a ceramic material prepared in accordance with the present invention.